What is rattiling inside my passenger door ?!

Have 2000 SS. Recently whenever I get a hard bump (and that is often, here in michigan), something inside my passenger door rattles loudly.
It never rattles on nice flat road, it is either a bump or up/down icline.
It rattles very fast - ratt-tat-tat-tat-tat-tat, etc and then stops.
I was thinking door arm or window guide? But the door, lock, window work fine. And I wouldn't have a clue how to get into the door and what to even look for.
Any help would be appreciated !
Thanks in advance.
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buzzsaw wrote:

Removal of the door panel is pretty straight forward. See http://shbox.com/1/4th_gen_tech1.html Select the How To's and Adjust the door window for reference. Be sure to pull up before pulling out on the panel.
Regards
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